[codeview] Fix buggy BeginIndexMapSize assertion

[codeview] Fix buggy BeginIndexMapSize assertion

This assert is just trying to test that processing each record adds
exactly one entry to the index map. The assert logic was wrong when the
first record in the type stream was a field list.

I've simplified the code by moving the LF_FIELDLIST-specific logic into
the callback for that record type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299035 91177308-0d34-0410-b5e6-96231b3b80d8